Molecular Gas Excitation in z ~ 0.7 Gas-Rich Post-starburst Galaxies from SQuIGGLE

TL;DR

This study investigates the molecular gas excitation in z~0.7 post-starburst galaxies, revealing moderate excitation levels and low star formation efficiency, with one active galactic nucleus showing higher excitation likely due to non-stellar heating.

Contribution

First detailed CO(5-4) observations of gas-rich post-starburst galaxies at z~0.7, linking molecular excitation to star formation activity and AGN presence.

Findings

Most galaxies have moderate CO excitation with r_{52} ~ 0.1-0.3.

Star formation surface densities are low, indicating suppressed star formation.

One galaxy with an AGN shows elevated excitation, likely due to non-stellar heating.

Abstract

Many post-starburst galaxies at $z\sim0.7$ have been shown to retain substantial molecular gas reservoirs yet host low ongoing star formation, suggesting that the remaining gas may be inefficient at forming stars during the early post-burst phase. We present new Atacama Large Millimeter/submillimeter Array CO(5-4) observations of nine gas-rich post-starburst galaxies at $z\sim0.7$ from the Studying Quenching in Intermediate-z Galaxies: Gas, angu$\vec{L}$ar momentum, and Evolution (SQuIGG$\vec{L}$E) survey, providing a view of the molecular gas excitation in these systems. Combined with existing CO(2-1) data, we detect CO(5-4) in 8/9 targets and find that most have moderate CO excitation with $r_{52}\equiv L'_{\rm CO(5-4)}/L'_{\rm CO(2-1)}\approx0.1-0.3$. These systems show no clear trend between $r_{52}$ and either total or surface-density of star formation. Specifically, all objects have $\Sigma_{\mathrm{SFR}} \sim 0.01-1\ \text{M}_\odot\ \text{yr}^{-1}\ \text{kpc}^{-2}$, consistent with compact, modest star formation, even when allowing for buried activity, as these galaxies decline from their peak. One object J1448+1010, which has clear optical, mid-infrared, and radio indicators of an active galactic nucleus, is an outlier with $r_{52}\approx0.6$; its elevated excitation likely requires significant non-stellar heating, with a contribution from potentially obscured star formation. Together, most gas-rich SQuIGG$\vec{L}$E post-starbursts have moderately excited molecular gas alongside little to modest star-forming activity, indicating that the remaining gas hosts relatively suppressed star formation efficiencies instead of strong buried starburst activity.